Japanese Patent Laying-Open No. 2004-14242 discloses a technique of diagnosing abnormality of a control relay (system main relay) in a power supply circuit. A power supply control system disclosed in this publication includes a battery, first and second relays connected to positive and negative electrode sides of the battery, respectively, and a voltmeter measuring a voltage across terminals of the first and the second relays each provided on a side opposite to the battery.
When the system is started up, with only one of the first and the second relays turned on, it is diagnosed whether or not the relay in an off state is welded, based on the voltage from the voltmeter.
However, Japanese Patent Laying-Open No. 2004-14242 does not consider an inrush current that may flow when a relay is turned on. The power supply control system disclosed in the above publication has no precharge circuit such as a limiting resistor for preventing an inrush current. Therefore, when one of the first and the second relays is turned on, if the other relay is welded, a circuit is formed between the battery and a capacitor, and an inrush current may flow therethrough. Thereby, even the normal relay may also become welded.
On the other hand, it is not preferable from the viewpoints of cost reduction and downsizing of the system and the like to provide a dedicated precharge circuit for precharging the capacitor.